It is well known that sun radiations ranging from 290 to 400 nm are noxious for the organic materials, among which cutaneous tissue too. In fact, prolonged exposition to sun radiation is considered to be the main cause of the development of degenerative processes and of skin cancer forms. In particular, radiations of wavelength between 290 and 320 nm, so called UV-B radiations, cause erythema and sunburns, whose severity depends on exposure length.
It was ascertained that also the radiations ranging between 320 and 400 nm, so called UV-A, and responsible of skin tanning, can cause alterations and damages in the skin which may not be disregarded, such as for example degenerative processes and even cancer forms; especially in case of sensible skins or in case of prolonged exposition to radiation.
It has also been demonstrated that the UV-A radiation other than causing damages to elastin and collagen, whose consequence is ageing of the skin, can also be the cause of a number of phototoxic and photoallergic reactions. Beside, the noxious action of UV-B may be enhanced by the presence of UV-A (see: Willis et al.: Journal of Investigative Dermatology vol. 59, 416, 1072).
By means of the use of particular compounds or of compositions containing these particular compounds, so called sunscreens, capable of absorbing, at least partially, UV sunlight radiations, noxious effects on organic materials, in particular on synthetic polymers and on human skin can be prevented or at least attenuated and ageing of the same slowed down. As protective agents a number of substances have been studied and experimented and a wide patent literature exists on this matter, in which compounds belonging to different chemical classes capable of absorbing in the UV zone of sun radiation and particularly that between 290 and 360 nm are proposed.
Many compounds, such as for example derivatives of cinnamic acid, 4-aminobenzoic acid, benzylydenecamphor, benzophenone, and diphenylcyanoacrylic acid are well known and widely used in cosmetic preparations for the protection from sunburns and erythema due to noxious UV-B radiation.
Until recently, the use of sunscreens for the protection from the UV-A radiation was practically unknown, other than some special cases of therapy. But recent studies show that also a continuous and intensive UV-A radiation can cause severe cutaneous damages, especially to persons having very sensitive and delicate skin.
For the protection against UV-A, really suitable products are not yet available, even if in the patent literature some compounds have been proposed, but in practice, the outcome of these compounds may not be considered sufficiently positive.
2-hydroxy-4-methoxybenzophenone is an often used commercial product, whose maximum absorption in the UV-A zone at about 325 nm, is too low to give an effective protection and, moreover its solubility in solvents usually used in cosmetics is very low thus making difficult its handling.
Another compound actually used in practice is a dibenzoylmethane derivative, but not only it is incompatible with many ingredients usually employed for cosmetic compositions, but also has the severe defect of not being sufficiently photostable (Int. J. Cosm. Science 10, 53 1988) the sun formulations containing these compounds may not guarantee a sufficient protection from UV-A since the sunscreens are either too weak (such as the benzophenone derivative) or are degraded too quickly by the radiation itself (such as the dibenzoylmethane derivative).
To date, therefore, it is not possible to satisfy the market requirements, since industry has not yet made available sunscreens capable of providing a sufficient protection from sunlight UV-A radiations.